In a single channel transmission system, for example a hands-free communication installation for telephones, a number of problems have already been addressed, which result from the coupling between loudspeaker and microphone, from the influence of the ambient noise and control of the duplex communication situation.
Thus, for example, a circuit arrangement for dynamic control of a terminal is known, where a controllable dynamic compander adapts the amplification factor of the signal voltages produced by a microphone to the voltage value of these signal voltages, see DE 37 24 346 A1. This achieves that several persons using the hands-free communication device of a subscriber affect the subscriber on the opposite side, as if they were located at the same distance from the terminal, and as if they were speaking at the same loudness, while interference noise sources barely have any effect on the transmission and the disturbing acoustical liveliness is avoided.
Furthermore, a method for improving the transmission properties of an electrical-to-acoustical installation is known, whereby the position of the compander curve is automatically controlled, see DE 42 29 912 A1. This detects whether the transmitted signal is derived from speech or from noise, and where the control magnitude produced by means of the voice and noise recognition for the compander curve achieves that the speech being transmitted is sent at a constant level, that the received loudness level is raised with increasing ambient noise, and that the noise level is lowered for the transmission.
A further improvement of the quality of hands-free communication is achieved, if the compander is preceded by a controlled echo compensator, or is followed by a voice-controlled balance, see DE 43 05 256 A1.
Finally, a method has already been proposed, whereby the current coupling between loudspeaker and microphone is determined, to attain the desired echo attenuation required by the system, with as little influence on the quality of the duplex communication as possible.
Although many problems are known with single channel hands-free communication, and have also been partly solved, the solutions cannot be used directly with multichannel transmission systems, since multichannel transmission systems must take multiple couplings into consideration between the loudspeakers and microphones in the same room. It is noted that n.sup.2 couplings take place with n-channel transmission systems. An arrangement for a two-channel transmission system is known, wherein an echo compensator is used for each channel, see Hirano A., Sugiyama A.: A Compact Multi-Channel Echo Canceller with a Single Adaptive Filter per Channel. Proc. ISCAS 1992, San Diego, Calif., 1992, pages 1922 to 1925. Each echo compensator is designed with an adaptive digital filter. The publication describes that when four adaptive filters are used, there is no clear optimum solution for the adjustment of the filter coefficients, to compensate for the echoes derived from the four possible couplings. The indicated solution with two digital filters is based on that one digital filter is switched on as a function of the input signal from the right and the left loudspeaker, and the pulse response at the right microphone is evaluated by the right and the left loudspeaker. However, the computation of the filter coefficient is only correct if a microphone receives signals from one loudspeaker only. The evaluation of composite or differential signals described herein gives rise to the danger of erroneous measurements, and can result in system instability. Furthermore, the use of digital filters in multichannel transmission systems is very costly, since for example nine filters must be figured for a three-channel system. For example, the required working speed of a computer with a scanning rate of 24 kHz and a filter length of 1000 coefficients, according to a time window of 41 ms, is 864 million instructions per second.
In addition, a multichannel transmission system is known, wherein one digital filter is used for each loudspeaker and microphone, see FP 627 825 A2. To compute the filter coefficients in that case, a detector circuit selects the channel in which the signal with the largest output occurs. To limit the expense of this solution, a compromise is made between the number of adaptive filters and the gain in quality through echo compensation.